Conventionally, so-called handling machines using lifting magnets (e.g., materials handling apparatus) have been widely used. The handling machine uses a powerful electromagnet so as to retainably attract magnetic members such as ferrous materials and then release the retainably attracting force at a location to which the members are transferred. For example, a typical conventional handling machine using a lifting magnet is one shown in FIG. 4. With reference to FIG. 4, the handling machine (its main body is not shown in FIG. 4) includes an engine 1. The engine 1 is provided in common, on a drive shaft thereof, with a main pump (hydraulic pump) 2 for supplying a pressurized working fluid to required hydraulic actuators, including each cylinder and each hydraulic motor on the machine main body side, and with a generator hydraulic pump 3. The discharge outlet of the generator hydraulic pump 3 is in communication with the pressurized fluid inlet of a generator hydraulic motor 4, and an electric generator 5 is directly coupled to the generator hydraulic motor 4.
The output terminal of the electric generator 5 is connected with a converter 6 for converting AC output of the electric generator 5 into DC output. The converter 6 is connected with a DC-DC converter 7 in a stage downstream of the converter 6. The DC-DC converter 7 converts the DC output, which has been obtained through the conversion by the converter 6, into a DC voltage output at a level required for energization of the lifting magnet device. The DC-DC converter 7 has a DC voltage step-up and step-down function as well as a switching function by which DC power remains unchanged (variation of DC power being zero) before and after step-up or step-down of a DC voltage. The output terminal of the DC-DC converter 7 is connected with a coil 8a of the lifting magnet device 8.
The DC-DC converter 7 is controlled by a controller 9 to perform conversions. Each of the components subsequent to the converter 6 is operated by turning ON or OFF a control switch (not shown) connected to the controller 9. Furthermore, a DC line 10 from the DC-DC converter 7 is connected with a large-capacitance capacitor 11 for accommodating energy to be stored in the coil 8a. 
On the other hand, the discharge outlet of the main pump 2 is in communication with the fluid supply port of a control valve 12 which has a direction switching function. The control valve 12 has a plurality of switching positions. Thus, an output port at one switching position is connected with a cylinder 13 used for a boom, an arm, a fork, or the like, while an output port at the other switching position is connected with a hydraulic motor 14 which is used for pivotal motion, rightward traveling, leftward traveling, or the like.
Then, the electric generator 5 is rotated by the engine 1 via the generator hydraulic pump 3 and the generator hydraulic motor 4 to generate alternate current. When the control switch connected to the controller 9 is turned ON, the converter 6 converts the AC output of the electric generator 5 into a DC output. Then, the DC output is in turn converted by the DC-DC converter 7 into a DC voltage at a required level to be supplied to (the coil 8a of) the lifting magnet device 8. Therefore, retainable attraction of objects is initiated.
As shown in FIG. 5, at the initiation of the retainable attraction, a voltage greater than a rated voltage is applied to the coil 8a of the lifting magnet device 8 for intense energization thereof. After a predetermined period of time has elapsed from the intense energization, steady-state energization is effected through application of a rated voltage. At the time of a release after the period of time of the steady-state energization, a termination of voltage application to the coil 8a causes the energy stored in the coil 8a to be accommodated by a condenser 11. After the termination of the application of the rated voltage to the coil 8a, a predetermined reverse voltage is applied thereto for demagnetization. After a predetermined period of time has elapsed from the initiation of the demagnetization, the application of the reverse voltage is terminated, thereby ending the lifting operation.
As a specific conventional technique related to the handling machine using a lifting magnet, a lifting magnet device is known which is disclosed, e.g., in the publication of Japanese Patent No. 3395145. This conventional technique includes a controller and a lifting magnet main body, and the controller is connected with an electrical power source for the handling machine. The electrical power source is an alternator serving as a standard electrical power source which is typically provided in a handling machine, and the alternator employed has a rated voltage of 24V DC and a rated capacity of 50 A. On the other hand, the rated voltage employed for the lifting magnet main body is the same as the rated voltage of the alternator. Thus, the controller is configured to supply a predetermined control voltage to the lifting magnet main body using the output from the electrical power source as input power. Such a configuration allows the conventional technique to dispense with a dedicated power source.
In the aforementioned conventional technique according to Japanese Patent No. 3395145, a so-called alternator at 24V DC for electrical components, which is normally provided in a handling machine, is used as an electrical power source to drive the lifting magnet main body. That is, this configuration can be said to regard the lifting magnet main body as one of the electrical components. However, the lifting magnet main body driven by 24V DC provides a weak retainably attracting force in practice, and in particular, cannot provide sufficient power for the intensely energized portion of FIG. 5.
Accordingly, retainably attracting force for practical use was obtained as follows. That is, as already discussed in the example of FIG. 4, the generator hydraulic pump installed on the drive shaft of the engine was typically used to drive the generator hydraulic motor, thereby driving the electric generator to obtain predetermined electric power.
However, this configuration caused problems such as low energy efficiency and tremendous increase in the size of the apparatus. In particular, by nature, the lifting magnet device needs to be ready all the time to be supplied with high electric power output so as to be intensely energized when starting a retainable attraction. To this end, it was necessary to prepare a corresponding large engine or for a slightly smaller engine to be rotated at high speeds all the time. Therefore, this readily causes problems such as increase in costs and size of the apparatus, decrease in energy efficiency, and increase in noise. Furthermore, with this configuration, it was also necessary to prepare a large-capacitance condenser for accommodating energy stored in the coil of the lifting magnet device. This also causes increase in the size of the energization-related components of the lifting magnet device.